This invention relates in general to earth boring bits, and particularly to features for reducing stress concentration between the bit legs and cutter bearing pins.
The most common earth boring bit for drilling relatively small diameter wells, such as oil and gas wells, have three rotatable cutters. Each cutter is mounted on a bearing pin that depends downwardly from a bit leg. During drilling, very large downward forces are exerted on the drill bit. This creates a high stress area at the junction of the bit leg with the bearing pin. Stress cracks may result on the inside of the bit leg and on the nonpressure or upper side of the bearing pin. This may result in shortening the life of the bit, and also result in having to fish out of the well broken components from the drill bit.
One type of bit uses a friction or journal bearing. In this type of bit, a flat annular surface is formed on the inner surface of the bit leg surrounding the cylindrical bearing pin. This annular surface, known as the "last machined surface", may be flush with the inside surface of the bit leg or part may be recessed into a groove. The last machined surface is located in a plane parallel with the plane of the cutter backface. The backface is an annular surface surrounding the cavity of the cutter, and located in a plane normal to the cutter axis.
The journal bearing bit of the prior art type normally uses an O-ring seal located in a fillet or seal boss formed at the intersection of the bearing pin and last machined surface. The area on the upper side at the bit leg--bearing pin juncture experiences the high stresses that are likely to result in cracks. The radius of the seal boss fillet is small, but cannot be increased substantially to reduce stress concentration, since it must equal the cross-sectional radius of the O-ring.
Also, in the prior art journal bearing bit, the section increases abruptly above and below the bit leg--bearing pin juncture. This abrupt change in section increases stress concentration at this point of juncture.